This invention relates generally to optical elements and more particularly to impact protection and strengthening of optical elements.
As is known in the art, optical imaging systems generally include externally mounted optical elements which shield the remainder of the imaging system from an external environment. For example, with infrared (IR) airborne imaging systems, an IR transparent optical element such as a window or dome is mounted on the airborne system to isolate the remainder of the IR imaging system from exposure to humid, corrosive and abrasive environments. Prolonged exposure to these environments generally degrade the optical and physical characteristics of the material of the optical element. Generally, the most severe environmental exposure encountered by these external elements appears to be high velocity water droplet impact which occurs when an airborne system is flown through a rain field.
This problem of water droplet impact is more generally referred to in the art as rain erosion. During flight through a rain field, water droplets impinge upon the surface of the external element producing subsurface fractures even at subsonic velocities. For very brittle materials these subsurface fractures are initiated at pre-existent microflaws lying near the surface of the optical element. Rain erosion damage to such optical elements occurs prior to any significant removal of material. The mere propagation of these pre-existent microflaws is sufficient to damage the optical element. In particular, these microflaws are propagated through the optical element by the tensile component of the surface stresswave created at the time of impact with the water droplet. Once formed, the continued propagation of the subsurface fractures through the optical element will often produce large cracks in the optical element. In the region of the crack, scattering and refraction of incident IR energy occurs producing increased internal reflections and IR energy losses. With a significant number of such cracks, the transmissivity of the optical element is severely reduced. Furthermore, as cracks propagate through the optical element, catastrophic failure of the element may occur. When the optical element shatters or breaks, the remaining optical elements of the IR imaging system are exposed to the external environment, resulting in potential catastrophic damage to the imaging system.
Typically, materials which offer the best mechanical durability and optical performance for infrared imaging systems, particularly in the 8 .mu.m to 12 .mu.m infrared band, are limited to a relatively small number. Suitable materials include zinc sulfide, zinc selenide, germanium, gallium arsenide, gallium phosphide, mercury cadmium telluride and cadmium telluride. Ternary sulfide materials such as calcium lanthanum sulfide are also currently being developed for IR applications, particularly in the 8-12 .mu.m band. These ternary sulfide materials may provide some improvement in durability but even these materials are susceptible to the environmental exposures mentioned above. Generally, all of the aforementioned materials are relatively brittle and have a relatively low resistance to damage, particularly damage sustained during high velocity water droplet impact.
It is also known in the art that optical energy incident upon a surface of an optical element will result in reflection of energy at such surface if the index of refraction of the material comprising the optical element is significantly different than the index of refraction of the medium from which the energy originates. Generally, for airborne systems, the originating medium is air having an index of refraction of about one. Accordingly, it is standard practice in the optical industry to provide coatings of material of appropriate refractive index over the incident surface of the optical element to reduce such reflection losses. At the deposited thicknesses, which are generally related to a fraction of an optical wavelength, these coatings are transparent in the IR band. However, heretofore such optical coatings have served only to reduce reflection losses caused by a mismatch in refractive indices and have not served to increase the impact resistance of the optical element.
It is known in the art that a layer of hard carbon, that is, a carbon layer having quasi-diamond bonds and substantial optical transparency, when provided over germanium provides limited protection to germanium optical elements from impact damage caused by rain erosion. Hard carbon coatings on germanium are described in an article entitled "Liquid Impact Erosion Mechanisms In Transparent Materials" by J.E. Fields et al, Final Report Sept. 30, 1982 to Mar. 31, 1983, Contract No. AFOSR-78-3705-D, Report No. AFWAL-TR-83-4101. The hard carbon surfaces have not successfully adhered to other IR materials such as zinc sulfide and zinc selenide. Furthermore, hard carbon coatings even on germanium as mentioned in the article are susceptible to debonding during high velocity water droplet impact. It was theorized there that the sheering force resulting from the radial outflow of water droplet impact causes debonding of the coating from the germanium layer. This phenomena of debonding is believed to significantly increase as the thickness of the hard carbon layer is increased. Therefore, thicker hard carbon coating layers which should have resulted in further impact protection for the optical element were not successful because of the aforementioned debonding problem. A further problem with hard carbon is that the index of refraction of hard carbon is about 2.45, substantially higher than the index refraction of many of the aforementioned optical materials such as zinc sulfide and zinc selenide. Accordingly, if an optical element is coated with a hard carbon coating, reflection losses at the incident surface of the optical element will be higher than if the optical element was not coated.
A third problem in the art concerns the fracture strength of these materials. Again, most materials which are suitable for IR transparent windows particularly in the 8 .mu.m to 12 .mu.m band have low fracture strengths. This characteristic is particularly important in applications of these elements where the element separates a high pressure region from a low pressure region, that is, in applications where the element is under some static or dynamic mechanical load. In an article entitled "Impact Damage Threshold In Brittle Materials Impacted By Water Drops" by A.G. Evans et al, Journal of Applied Physics 51 (5), pps. 2473-2482 (May, 1980) at page 2481 it was theorized that martensite toughening (phase changes) at the surface of the brittle material may be useful in tempering such brittle materials. It was also theorized that surface compression stresses could be of benefit. However, the authors gave no specific description what they meant by "surface compression." These brittle materials undergo surface compression when incident water drops impact the surface of the material.